Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Operating revenues	$ 301,207	$ 278,828
Operating expenses:
Cost of revenues	67,521	63,172
Selling and marketing	43,014	41,689
Research and development	18,977	18,928
General and administrative	21,004	21,890
Amortization of intangible assets	11,251	11,840
Depreciation and amortization of property, equipment and leasehold improvements	8,838	8,168
Total operating expenses	170,605	165,687
Operating income	130,602	113,141
Interest income	(932)	(621)
Interest expense	29,024	22,904
Other expense (income)	885	81
Other expense (income), net	28,977	22,364
Income before provision for income taxes	101,625	90,777
Provision for income taxes	28,674	30,410
Net income	$ 72,951	$ 60,367
Earnings per basic common share	$ 0.80	$ 0.61
Earnings per diluted common share	$ 0.80	$ 0.60
Weighted average shares outstanding used in computing earnings per share
Basic	90,708	99,425
Diluted	91,624	99,998
Dividend declared per common share	$ 0.28	$ 0.22